Ferguson, Missouri. — The protests here were calm and orderly Monday evening, until the sun went down and things heated up.

During the day, protesters walked in a line on the sidewalk along West Florissant Street, but at night they moved into the streets and marched up the road. Police formed a line and engaged in a tense standoff with protesters at the intersection of Ferguson Avenue and West Florissant Street. After a few unruly protesters began hurling water bottles and other items at police, the cops removed three protesters right away for inciting a riot. Protesters grabbed one man and pushed him forward toward the police line, and turned him in to police.

In the chaos, several officers appeared ready to make a move, but others ordered them to stand down. Police eventually responded to violence that escalated further down the road, and ordered people in the area to leave immediately. At an early Tuesday morning press conference, Missouri Highway Patrol captain Ron Johnson, who’s in charge of the policing situation, said 31 people had been arrested — coming from places as far away as California and New York — and two people had been shot. It remains unclear who fired the shots, but NBC News has reported that approximately 78 people were arrested last night.

But the events in Ferguson are beginning to form a pattern. During the day, people protest in a non-violent fashion that makes it seem as though things are finally calming down. When the sun sets, people grow violent and turn on police. After multiple days of this pattern, it’s difficult to understand why police have not yet found a way to contain the violence on the ground.

Police officers had prevented people from congregating in any one spot all day long and forced protesters to stick to the sidewalks and keep moving. But as it grew darker, protesters and police alike appeared to grow more tired. Protesters stopped and sat down in parking lots and police did not chase them away. One officer told me he had worked ten consecutive days and had to miss the birthday parties of his niece and nephew.

Protesters seemed noticeably weary too, as they marched all day in the hot sun. African-American pastors passed out water as Nelly, a rapper, used a megaphone to tell protesters not to do anything stupid. He implored protesters to develop a strategic plan to remedy the situation. Others encouraged the protesters to vote, not fight. Protesters resumed their march after hearing from Nelly, and soon the number of marchers grew. They moved into the street sometime later, against the orders of police who had succeeded in keeping the protests out of the roadway all day long. Their march prompted the standoff with police officers.

It remains difficult to determine why police seem to put more effort into keeping people out of the streets during the daytime than at night. Police handcuffed two photographers standing still on a sidewalk in the afternoon, but allowed a massive crowd of protesters to gather in the street at night. The temperature in Ferguson is rising — it will be in the upper 90s for the rest of the week in the St. Louis suburb — alongside growing tension between police and protesters.

“You just wait until a trial begins,” one protester shouted on Ferguson Avenue in the direction of police. “That’s when this gets bad.”